As in this case a large part of the construction work was finished, the actual number of accidents in the construction department dropped to almost nothing, and, because the shaded area for construction grew less, it was necessary that the other areas should widen out if the 100 per cent line at the top were to remain a straight line. Here the weak point in the method of charting is the same as that indicated for Fig. 128. The person observing the chart has no way of telling whether the factors included in the 100 per cent have grown less or grown greater, and whether

COMPONENT PARTS SHOWN BY CURVES

Percent too

the quantities represented by the different areas have changed in actual size or only in relative size. Where great fluctuations occur from time to time and many factors enter into the total, it is best to draw charts in the form of Fig. 129 with a common zero line, or in the form of Fig. 131, where each factor has its own separate base line, or in the form of Fig. 132 and Fig. 133, in which the lines on the chart represent actual quantities rather than percentages of an aggregate or total sum.

Note in Fig. 130 and Fig. 131 the 8V^-inch by 11-inch co-ordinate paper on which the ruling is so arranged that the paper may be used for almost any subdivisions of time, such as days, weeks, months, etc., as seen in Figs. 57, 103. 131, 134, and 156.